A$12.$ A rectangular channel is required to carry a discharge of $8\frac{m^3}{s}.$ If the

bed slope is $0\mathrm{.}0001$ and the Manning's $n=0\mathrm{.}01,$ what should be the

dimensions of a cross$-$section to give maximum discharge?

a$)y=1m$;$b=2m$

b$)y=1m$;$b=4m$

c$)y=2m$;$b=2m$

d$)y=2m$;$b=4m$

e$)y=4m$;$b=2m$